Sort the spinach, wash and drain. Wash and halve the tomatoes. Season vinegar with salt, pepper and sugar, fold in 2 tablespoons of oil
Wash the meat, dab dry. Brush a grill pan with 1 teaspoon of oil. Fry the meat in it from both sides for about 10 minutes, season with salt and pepper, take it out and let it cool down. Peel and finely dice the onion. Heat 1 tsp. oil in a saucepan. Sauté the onion in it. Add tomato paste and sweat for 1-2 minutes. Mix with 75 ml water to a thick sauce, season with salt and pepper
Shred mozzarella. Place 2 slices of bread on a microwaveable plate and cover with mozzarella. Heat in the microwave at approx. 600 watts for 20-25 seconds until the cheese has melted
Cut the meat open, cover slices of bread with it, drizzle with sauce and cover with spinach. Cover with slices of bread and pin with wooden skewers. Mix the remaining spinach, tomatoes and vinaigrette and serve as well